Varactor diode with doped voltage blocking layer

ABSTRACT

A varactor diode includes a contact layer having a first conductivity type, a voltage blocking layer having the first conductivity and a first net doping concentration on the contact layer, a blocking junction on the voltage blocking layer, and a plurality of discrete doped regions in the voltage blocking layer and spaced apart from the carrier injection junction. The plurality of discrete doped regions have the first conductivity type and a second net doping concentration that is higher than the first net doping concentration, and the plurality of discrete doped regions are configured to modulate the capacitance of the varactor diode as a depletion region of the varactor diode expands in response to a reverse bias voltage applied to the blocking junction. Related methods of forming a varactor diode are also disclosed.

FIELD OF THE INVENTION

The present invention relates to electronic devices, and in particularto varactor diodes for use in load modulation schemes for improvedamplifier efficiency.

BACKGROUND

High frequency amplifiers operate most efficiently when the output ofthe amplifier is delivered to an impedance matched load, that is, a loadthat has an input impedance equal to the output impedance of theamplifier. However, the output impedance of a high frequency amplifieris a function of both the frequency of operation of the amplifier aswell as the power output by the amplifier. Thus, to obtain highefficiency at a given operating frequency, it is desirable to adapt theimpedance of the load based on the power output by the amplifier. Forexample, some digital modulation techniques have a high peak-to-averagepower ratio. A designer of an RF power module may choose to optimize theamplifier chain for efficiency at the peak output power. However, inthat case, the efficiency of the amplifier at the average power levelmay suffer.

Varactor diodes, also referred to as varicap diodes, variablecapacitance diodes, and tuning diodes, can be used as variable reactanceelements in an impedance matching transformer for a high frequencyamplifier. The capacitance of a varactor diode can be controlled byadjusting the reverse bias level of the diode. Because varactor diodesare operated in reverse bias, only limited current flows through thediode during such operation. However, since the thickness of thedepletion region in a reverse biased diode varies with the applied biasvoltage, the capacitance of the diode can be controlled. In aconventional diode structure, the depletion region thickness isproportional to the square root of the applied voltage, and thecapacitance of the diode is inversely proportional to the depletionregion thickness. Thus, the capacitance of a conventional diode isinversely proportional to the square root of applied voltage.

The performance of a varactor is also characterized by its so called Qfactor, defined as the ratio between the capacitive reactance to theequivalent series resistance (ESR). To achieve high Q (i.e., low lossperformance), the material used to form the varactor should be able tosupport a high control voltage while maintaining low resistance. Thewide bandgap materials including, but not limited to SiC or GaN, have aninherently high electric field strength, allowing a thin highly dopedlayer to have a large breakdown voltage. Accordingly, such materialsshould allow a very high Q value to be achieved with low insertion loss.

Some attempts have been made to alter the capacitance-voltagerelationship of silicon or gallium arsenide-based varactor diodes byproviding a graded doping profile in the voltage blocking region of thediode. The graded doping profile has been typically achieved eitherusing epitaxial growth, diffusion or implantation. However, providing acomplex graded doping profile can prove difficult in practice, becauseof the limitations of the processes used in reproducible manufacturing.

SUMMARY

A varactor diode according to some embodiments includes a highly dopedcontact layer that may include the substrate of the device, a voltageblocking layer having a first conductivity type and a first net dopingconcentration on the contact layer, a blocking junction on the voltageblocking layer, and a plurality of discrete doped regions in the voltageblocking layer and spaced apart from the carrier injection junction. Thediscrete doped regions have the first conductivity type and a second netdoping concentration that is higher than the first net dopingconcentration. The discrete doped regions are configured to modulate thecapacitance of the varactor diode as the depletion region of thevaractor diode expands in response to a reverse bias voltage appliedacross the blocking junction. By using a number of parallel coupleddiodes, each containing discrete, well-defined doping regions, a complexgraded doping profile can be approximated.

The plurality of discrete doped regions may include first discrete dopedregions spaced a first distance from the blocking junction and seconddiscrete doped regions spaced a second distance from the carrierinjection junction, the second distance is greater than the firstdistance.

The first discrete doped regions may provide a first total charge at thefirst distance from the blocking junction and the second discrete dopedregions may provide a second total charge at the second distance fromthe blocking junction that is greater than or equal to the first totalcharge.

The first discrete doped regions may have a first net dopingconcentration and the second discrete doped regions may have a secondnet doping concentration that is less than the first net dopingconcentration.

The plurality of discrete doped regions may further include thirddiscrete doped regions spaced a third distance from the blockingjunction that is greater than the second distance, and the thirddiscrete doped regions may have a third net doping concentration that isless than the second net doping concentration.

The plurality of discrete doped regions may further include thirddiscrete doped regions spaced a third distance from the blockingjunction that is greater than the second distance.

The first, second and third discrete doped regions may not necessarilyoverlap in a lateral direction parallel to the carrier injectionjunction.

The varactor diode may further include a Schottky metal contact on thevoltage blocking layer, and the blocking junction may include a Schottkybarrier junction.

The varactor diode may further include a second layer having a secondconductivity type opposite the first conductivity type on the voltageblocking layer, and the blocking junction is thereby formed by a P-Njunction between the second layer and the voltage blocking layer.

The voltage blocking layer may include a wide bandgap semiconductormaterial such as silicon carbide.

Some embodiments of the invention provide a variable impedance matchingcircuit for a high frequency amplifier including a varactor diode asdescribed above.

Methods of forming a varactor diode according to some embodiments of theinvention include providing a voltage blocking layer having a firstconductivity type and a first net doping concentration, and forming aplurality of discrete doped regions in the voltage blocking layer andspaced apart from a blocking junction. A blocking junction is formed onthe voltage blocking layer. The discrete doped regions have the firstconductivity type and a second net doping concentration that is higherthan the first net doping concentration, and the plurality of discretedoped regions are configured to modulate the capacitance of the varactordiode as a depletion region of the varactor diode expands in response toan applied reverse bias voltage.

Forming the plurality of discrete doped regions may include implantingions into the voltage blocking layer. In particular, forming theplurality of discrete doped regions may include selectively implantingfirst ions at a first implant energy and first dose to form firstdiscrete doped regions spaced a first distance from the blockingjunction and selectively implanting second ions at a second implantenergy and second dose to form second discrete doped regions spaced asecond distance from the blocking junction that is greater than thefirst distance.

Selectively implanting first ions and second ions may includeselectively implanting the first ions and the second ions so that thefirst and second discrete doped regions do not overlap in a lateraldirection parallel to the carrier injection junction.

The first discrete doped regions may provide a first total charge at thefirst distance from the blocking junction and the second discrete dopedregions may provide a second total charge at the second distance fromthe blocking junction that is greater than or equal to the first totalcharge.

The first discrete doped regions may have a first net dopingconcentration and the second discrete doped regions may have a secondnet doping concentration that is less than the first net dopingconcentration.

Implanting ions into the voltage blocking layer may include forming amulti-level implant mask on the voltage blocking layer and implantingions into the voltage blocking layer through the multi-layered implantmask so that ions implanted through the multi-level implant mask arepositioned at different depths in the voltage blocking layercorresponding to different thicknesses of the multi-level implant mask.

A varactor diode structure according to some embodiments includes acommon contact layer having a first conductivity type, a first voltageblocking layer having the first conductivity type and a first net dopingconcentration on the common contact layer, a first blocking junction onthe first voltage blocking layer, and a first plurality of discretedoped regions in the first voltage blocking layer and spaced apart fromthe first blocking junction. The first plurality of discrete dopedregions have the first conductivity type and a second net dopingconcentration that is higher than the first net doping concentration.The structure further includes a second voltage blocking layer havingthe first conductivity type and a third net doping concentration on thecommon contact layer and spaced apart from the first voltage blockinglayer, a second blocking junction on the second voltage blocking layer,and a second plurality of discrete doped regions in the second voltageblocking layer and spaced apart from the second blocking junction. Thesecond plurality of discrete doped regions have the first conductivitytype and a fourth net doping concentration that is higher than the thirdnet doping concentration. An ohmic contact is on the common contactlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate certain embodiment(s) of theinvention. In the drawings:

FIG. 1 is a cross-sectional view of a Schottky diode structure accordingto some embodiments of the invention.

FIG. 2 is a cross-sectional view of a PIN diode structure according tosome embodiments of the invention.

FIG. 3 is a cross-sectional view illustrating fabrication of a Schottkydiode structure according to some embodiments of the invention.

FIGS. 4A and 4B are cross-sectional views of Schottky diode structuresaccording to further embodiments of the invention.

FIG. 5 is a circuit diagram of a high-frequency amplifier including avariable impedance matching circuit according to some embodiments of theinvention.

FIG. 6 a cross-sectional view of a structure according to someembodiments of the invention including a pair of Schottky diodesconnected in an anti-series configuration.

DETAILED DESCRIPTION

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below,” “above,” “upper,” “lower,” “horizontal,”“lateral,” “vertical,” “over,” “beneath,” “on,” etc., may be used hereinto describe a relationship of one element, layer or region to anotherelement, layer or region as illustrated in the figures. It will beunderstood that these terms are intended to encompass differentorientations of the device in addition to the orientation depicted inthe figures.

Embodiments of the invention are described herein with reference tocross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be exaggeratedfor clarity. Additionally, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, embodiments of theinvention should not be construed as limited to the particular shapes ofregions illustrated herein but are to include deviations in shapes thatresult, for example, from manufacturing. For example, an implantedregion illustrated as a rectangle will, typically, have rounded orcurved features and/or a gradient of implant concentration at its edgesrather than a discrete change from implanted to non-implanted regions.Likewise, a buried region formed by implantation may result in someimplantation in the region between the buried region and the surfacethrough which the implantation takes place. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the actual shape of a region of a device andare not intended to limit the scope of the invention.

Some embodiments of the invention are described with reference tosemiconductor layers and/or regions which are characterized as having aconductivity type such as n-type or p-type, which refers to the majoritycarrier concentration in the layer and/or region. Thus, n-type materialhas a majority equilibrium concentration of negatively chargedelectrons, while p-type material has a majority equilibriumconcentration of positively charged holes. Some material may bedesignated with a “+” or “−” (as in n+, n−, p+, p−, n++, n−−, p++, p−−,or the like), to indicate a relatively larger (“+”) or smaller (“−”)concentration of majority carriers compared to another layer or region.However, such notation does not imply the existence of a particularconcentration of majority or minority carriers in a layer or region.

Referring to FIG. 1, a Schottky diode structure 10A according to someembodiments of the invention is illustrated. The diode 10A includes acontact layer 12 which can be highly doped and a voltage blocking layer14, which can be lightly doped, on the contact layer 12. In someembodiments, the contact layer 12 and the voltage blocking layer 14 aren-type; however, the conductivity types of the various layers describedherein could be reversed. In some embodiments, the contact layer 12 mayhave a net doping concentration of about 1×10¹⁷ cm⁻³ to about 5×10¹⁹cm⁻³, while the voltage blocking 14 layer may have a net dopingconcentration of about 1×10¹⁵ cm⁻³ to about 2×10¹⁸ cm⁻³. In particularembodiments, the contact layer 12 may have a net doping concentration ofabout 5×10¹⁸ cm⁻³ to about 5×10¹⁹ cm⁻³, while the voltage blocking 14layer may have a net doping concentration of about 2×10¹⁶ cm⁻³ to about2×10¹⁸ cm⁻³. The thickness of the voltage blocking layer 14 may beselected to provide a desired blocking voltage level.

A metal anode contact 16 forms a Schottky junction J1 with the voltageblocking layer 14 in an active region defined by a field dielectric 18.The metal contact 16 may include a metal such as aluminum, titaniumand/or nickel, which may form a Schottky junction with a silicon carbidelayer. A cathode contact (not shown) is formed on the contact layer 12.An edge termination (not shown), such as a field plate, floating guardrings, and/or a junction termination extension can be formed around thejunction to reduce field crowding at the periphery of the junction.

In some embodiments, the contact layer 12 comprises a bulk singlecrystal of a wide bandgap semiconductor, such as silicon carbide,diamond or a Group-III nitride material, such as gallium nitride,aluminum gallium nitride, etc. In the case of silicon carbide, thecontact layer 12 may have a polytype selected from 3C, 4H, 6H, and 15R,and may have an orientation that is on-axis or slightly off-axis. Inparticular embodiments, the contact layer 12 includes 4H- or 6H siliconcarbide having an off-axis orientation of up to 8° off-axis. As usedherein, a “wide bandgap semiconductor” refers to a semiconductormaterial having a bandgap greater than about 2.5 eV, and includes atleast silicon carbide and the Group-III nitrides.

Silicon carbide may be a particularly good semiconductor material foruse in embodiments of the present invention. Silicon carbide (SiC) hasbeen known for many years to have excellent physical and electronicproperties which allow production of electronic devices that can operateat higher temperatures, higher power and/or higher frequency thandevices produced from silicon (Si) or GaAs. The high electric breakdownfield of above 2×10⁶ V/cm, high saturated electron drift velocity ofabout 2×10⁷ cm/sec and high thermal conductivity of about 4.9 W/cm-Kindicate that SiC is suitable for high frequency, high powerapplications. In particular, the wide bandgap (about 3.0 eV for 6H—SiC)and high electric breakdown field of SiC makes SiC an excellent choicefor high voltage devices. As used herein, “high frequency” refers tofrequencies greater than about 30 MHz.

In particular, SiC-based devices are capable of blocking substantiallygreater voltages than comparable GaAs or Si devices for a giventhickness of the voltage blocking, or drift, layer. Thus, for example, aSiC Schottky diode having a drift layer thickness of 2 μm istheoretically capable of sustaining a voltage greater than 400V withoutbreakdown, while a similar device in silicon would require a drift layerthickness about 10 times as large. A varactor diode 10A according tosome embodiments may have a voltage rating of about 50 to 200 V.Accordingly, in embodiments in which the drift layer 14 comprisessilicon carbide, the drift layer 14 may have a thickness of less than 2μm.

In some embodiments, the interface between the contact layer 12 and thevoltage blocking layer 14 may include a heterojunction (i.e. a junctionbetween dissimilar materials). For example, in some embodiments, thecontact layer 12 may include a bulk single crystal of silicon carbide,and the voltage blocking layer 14 may include an epitaxial layer of aGroup III-nitride, such as GaN. However, in some embodiments, theinterface between the contact layer 12 and the voltage blocking layer 14may include a homojunction (i.e. a junction between similar materials).

The varactor diode 10A further includes a plurality of discrete dopedregions 20A-20C embedded in the voltage blocking layer 14. The discretedoped regions 20A-20C have the same conductivity type as the voltageblocking layer 14, but have different doping concentrations compared tothe voltage blocking layer 14. In particular, the discrete doped regions20A-20C may comprise delta-doped n+ regions in an n− voltage blockinglayer 14. The discrete doped regions 20A-20C may have a net dopingconcentration in the range of about 1×10¹⁶ cm⁻³ to about 5×10¹⁹ cm⁻³. Inparticular embodiments, the discrete doped regions 20A-20C may have anet doping concentration in the range of about 1×10¹⁸ cm⁻³ to about5×10¹⁹ cm⁻³. The doping concentrations of the discrete doped regions20A-20C may be selected to provide a desired capacitance-voltagerelationship when the diode 10A is reverse biased.

The discrete doped regions 20A-20C may not be interconnected with oneanother. In particular, there may be no interconnection between thediscrete doped regions 20A at a first depth in the voltage blockinglayer 14 and discrete doped regions 20B at a second depth in the voltageblocking layer 14, or between discrete doped regions 20B at the seconddepth in the voltage blocking layer 14 and discrete doped regions 20C ata third depth in the voltage blocking layer 14. In some embodiments,discrete doped regions 20A-20C at the same depth in the voltage blockinglayer 14 may be connected together.

In some embodiments, the doping concentration of the discrete dopedregions 20A-20C may vary with depth (i.e. distance from the junctionJ1). For example, in some embodiments, the doping concentration of thediscrete doped regions 20A-20C may decrease with distance from thejunction J1. In other embodiments, the doping concentration of thediscrete doped regions 20A-20C may increase with distance from thejunction J1. In still other embodiments, the doping concentration of thediscrete doped regions 20A-20C may be about the same.

In some embodiments, similar implant doses may be used to form thediscrete doped regions 20A-20C, so that the total charge of each of theregions may be about the same. However, because more deeply implanteddopants tend to spread out more (i.e. the straggle of the implant tendsto increase with implant depth), the more deeply implanted dopants maybe distributed with a lower concentration in terms of atoms/cm³.

Using ion implantation techniques to form the discrete doped regions20A-20C may permit placement of the discrete doped regions 20A-20C atany depth in the voltage blocking layer 14 tip to about 0.5 μm, based onthe maximum implantation energy used and the species implanted. Althoughthree different depths of discrete doped regions 20A-20C are illustratedin FIG. 1, it will be appreciated more or fewer levels of discrete dopedregions 20A-20C can be provided. The number and/or positioning of thediscrete doped regions 20A-20C may be selected to provide a desiredcapacitance-voltage relationship when the diode 10A is reverse biased.

The discrete doped regions 20A-20C can have a vertical thickness of fromabout 10 to 50 nm depending on the particular implant energy used. Asused herein, “thickness” of the discrete doped regions 20A-20C refers tothe vertical extent of a region of doped material in the voltageblocking layer 14 having a doping concentration in excess of 1×10¹⁵cm⁻³.

In some embodiments, the total charge provided by the discrete dopedregions 20A-20C can vary with depth in the voltage blocking layer 14.For example, the total charge provided by the discrete doped regions20A-20C can decrease or increase with depth, depending on the desiredcapacitance-voltage relationship of the diode 10A. The total charge canbe varied by varying the doping concentration in the discrete dopedregions 20A-20C based on depth, and/or varying the number, size, and/ordensity of discrete doped regions 20A-20C in the voltage blocking layer14 based on depth.

As the depletion region of the diode 10A expands outward from thejunction J1 with increased reverse bias, the depletion regionsuccessively encloses the discrete doped regions 20A-20C, and thecapacitance of the diode 10A changes accordingly. Thus, thecapacitance-voltage relationship of the diode can be engineered byappropriate placement and doping of the discrete doped regions 20A-20C.

As illustrated in FIG. 1, the discrete doped regions 20A-20C may notoverlap one another in a lateral direction (i.e. parallel to thejunction J1) in some embodiments. In particular, manufacturingtechniques using a single implant step may result in formation ofdiscrete doped regions 20A-20C that do not overlap one another in alateral direction. However, in some embodiments, the discrete dopedregions 20A-20C may overlap one another in a lateral direction.

The presence of the discrete doped regions 20A-20C varies thecapacitance-voltage relationship of the diode 10A. In particular, thedepth, doping and/or layout of the discrete doped regions 20A-20C in thediode 10A can be selected to provide a desired capacitance-voltagerelationship for the diode 10A. The diode structure 10A illustrated inFIG. 1 provides a plurality of parallel diode structures that havedifferent effective capacitances with delta doped profiles at differentdepths. Accordingly, a varactor diode having a high dynamic range ofcapacitance can be formed by providing the discrete doped regions20A-20C.

The discrete doped regions 20A-20C can be formed, for example, as deltadoped regions in the voltage blocking layer 14 via selectiveimplantation of ions. In particular, n-type ions, such as nitrogenand/or phosphorus, can be implanted into silicon carbide at implantationenergies up to about 500 keV, corresponding to a maximum implant depthof up to about 0.5 μm. Although higher implant energies are possible,implantation at higher energies may result in unacceptably high levelsof straddle, which can cause the discrete doped regions 20A-20C tospread out too much in a vertical direction.

Due to the high electric field strength of silicon carbide, the discretedoped regions 20A-20C can be formed entirely within the silicon carbidevoltage blocking layer at depths at which they can affect thecapacitance-voltage characteristics of the diode 10A under normaloperating conditions.

Implantation of ions into silicon carbide is well known to those skilledin the art and is described, for example in U.S. Publication No.2007/0292999, U.S. Pat. No. 5,851,908, and U.S. Pat. No. 6,107,142,which are assigned to the assignee of the present invention and thedisclosures of which are incorporated herein by reference.

After implantation of dopants into the voltage blocking layer 14 to formthe discrete doped regions 20A-20C, an activation anneal may beperformed at a temperature in a range of about 1400° C. to about 1700°C. for between about 5 minutes and about 30 minutes.

For wide bandgap materials, the large field strength that can besupported means that the discrete doped regions 20A-20C can be placedthroughout the voltage blocking layer 14 using ion implantation. Incomparison, in the case of Si or GaAs based devices having comparableblocking voltages, the implantation depths achievable in such materialswould only allow a fraction of the total thickness of the voltageblocking layer to be implanted. Since the voltage blocking layerthickness is comparable to that of the complex doped region, no excessthickness may be necessary to support voltage and add thereby add to theseries resistance of the device.

Referring to FIG. 2, a PIN diode structure 10B according to someembodiments is illustrated. Like numbers refer to like elements. In thePIN diode structure 10B, a highly doped layer 22 having an oppositeconductivity type from the voltage blocking layer 14 is formed on thevoltage blocking layer 14, and forms a P-N junction J2 with the voltageblocking layer. An ohmic contact 26 is formed on the doped layer 22.Modulation of the capacitance of the PIN diode structure 10B in responseto a reverse bias is similar to that of the Schottky diode structure 10Aof FIG. 1. In particular, a plurality of discrete doped regions 20A-20Care formed in the voltage blocking layer 14. As the depletion region ofthe diode 10B expands outward from the junction J2 with increasedreverse bias, the depletion region successively encloses the discretedoped regions 20A-20C, and the capacitance of the diode 10B changesaccordingly.

FIG. 3 illustrates operations of forming varactor diodes according tosome embodiments. As shown therein, a plurality of discrete dopedregions 20A-20C can be formed in a voltage blocking layer 14 using asingle implantation step. A multi-layered implant mask 60 is formed on asurface of the voltage blocking layer 14. The multi-layered implant mask60 includes a plurality of first steps 60A having a first thickness andcorresponding to the first discrete doped regions 20A (i.e. theshallowest regions), and a plurality of second steps 60B having a secondthickness that is less than the first thickness and corresponding to thesecond discrete doped regions 20B. The multi-layered implant mask 60further includes at least one opening 60C corresponding to the thirddiscrete doped region 60C (the deepest region).

When ions 70 are implanted into the structure in a single implant step,the portions of the multi-layered implant mask 60 having a firstthickness t1 block some ions completely. However, the ions implantedinto the portions of the mask having a second thickness t2, less thant1, are permitted to penetrate to a first depth d1 in the voltageblocking layer 14 (corresponding to the first discrete doped regions20A). Other ions implanted into the portions of the mask having a thirdthickness t3, less than t2, are permitted to penetrate to a second depthd2, which is greater than d1, in the voltage blocking layer 14(corresponding to the second discrete doped regions 20B). Yet other ionspass through the opening 60C in the mask 60 and are permitted topenetrate to a third depth d3, which is greater than d2, in the voltageblocking layer 14 (corresponding to the third discrete doped regions20C). In this manner, each of the discrete doped regions 20A-20C can beformed in a single implantation step, thereby potentially reducingmanufacturing cost and/or time.

It will be appreciated that the discrete doped regions 20A-20C could beformed using separate mask and implant steps with different implantenergies. The implant energies and doses are selected to provide adesired configuration of discrete doped regions 20A-20C in the voltageblocking layer 14.

FIGS. 4A and 4B are cross-sectional views of Schottky diode structures10C, 10D according to further embodiments of the invention. Referring toFIG. 4A, the discrete doped regions 20A′-20C′ in the Schottky diodestructure 10C can have a common size and doping level, but the densityand/or number of discrete doped regions provided in the voltage blockinglayer 14 can vary with distance from the junction J1. For example, thefirst discrete doped regions 20A′ are provided at a first distance d1from the junction, the second discrete doped regions 20B′ are providedat a second distance d2 from the junction where d2>d1, and the thirddiscrete doped regions 20C′ are provided at a third distance d3 from thejunction where d3>d2. Furthermore, the first discrete doped regions 20A′are provided at an areal density that is higher than an areal density ofthe second discrete doped regions 20B′. Likewise, the second discretedoped regions 20B′ are provided at an areal density that is higher thanan areal density of the third discrete doped regions 20C′.

Referring to FIG. 4B, the discrete doped regions 20A″-20C″ in theSchottky diode structure 10D are provided at varying depths in thevoltage blocking layer 14. In the diode structure 10D, however, thediscrete doped regions 20A″-20C″ overlap in the lateral direction. Thedoped regions 20A″-20C″ may have net doping concentrations and/orthicknesses that vary with depth. For example, the net dopingconcentrations of the doped regions 20A″-20C″ may decrease with distancefrom the junction J1.

FIG. 5 is a circuit diagram of a high-frequency amplifier circuitincluding an amplifier 30 that drives a high-frequency signal into aload 34. A variable impedance matching circuit 40 according to someembodiments of the invention is coupled between the amplifier 30 and theload 34, and adjusts the input impedance of the load 34 as seen by theamplifier 30. The variable impedance matching circuit 40 includes a pairof varactor diodes 10′, 10″ according to embodiments of the inventionconnected in an anti-series configuration with commonly connectedcathodes. A control terminal 42 provides a reverse bias voltage to bothof the varactor diodes 10′, 10″, and is used to modulate the capacitanceof the diodes. It will be appreciated that the variable impedancematching circuit 40 can include other reactive elements configured toprovide a desired impedance match. Furthermore, the varactor diodes canbe provided in a different configuration, for example including furtherdiodes in series so as to extend the capacitance tuning range.Accordingly, the circuit of FIG. 5 is provided as an exemplaryimplementation only.

FIG. 6 illustrates a structure 50 including a pair of varactor diodes10′, 10″ according to further embodiments of the invention. The varactordiodes 10′, 10″ include respective voltage blocking layers 14 on acommon contact layer 30, and Schottky contacts 16 on the voltageblocking layers 14. However, it will be appreciated that one or more ofthe diodes 10′, 10″ could include a PIN diode structure. The commoncontact layer 30 is supported by a semi-insulating substrate 32, whichmay include semi-insulating silicon carbide. Other insulating orsemi-insulating materials may be used for the substrate 32, such as AlN,alumina, sapphire, etc.

The common contact layer 30 may have the same conductivity type as thevoltage blocking layers 14, and will have a higher doping concentrationthan the voltage blocking layers 14. In some embodiments, the commoncontact layer 30 may include a bulk single crystal of silicon carbide oranother wide bandgap semiconductor, and the voltage blocking layers 14may include epitaxial layers of silicon carbide or another wide bandgapsemiconductor. In some embodiments, the common contact layer 30 mayinclude a bulk single crystal of silicon carbide, and the voltageblocking layers 14 may include epitaxial layers of a Group III-nitride,such as GaN, so that the interfaces between the common contact layer 30and the voltage blocking layers 14 may form heterojunctions. However, insome embodiments, the common contact layer 30 and the voltage blockinglayers 14 may form homojunction interfaces.

As the diodes 10′, 10″ share the common contact layer 12, the diodes10′, 10″ are connected in an anti-series configuration. Accordingly, anohmic contact 42 on the common contact layer 30 can be used to provide abias voltage for both diodes 10′, 10″.

A plurality of discrete doped regions 20 are formed in the respectivevoltage blocking layers 14 in the manner described above to provide thediodes 10′, 10″ with appropriate capacitance modulation characteristics.The discrete doped regions in the diodes 10′, 10″ may be formed to havethe same or different capacitance modulation characteristics.

Although embodiments of the invention have been described as beingintended for use in high efficiency RF power amplifiers, varactor diodesaccording to some embodiments can be used in other applications in whicha voltage controlled impedance is used, such as voltage controlledoscillators (VCOs), tunable filters, phase shifters, active antennas,etc.

In the drawings and specification, there have been disclosed typicalembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

1. A varactor diode, comprising: a contact layer having a firstconductivity type; a voltage blocking layer on the contact layer, thevoltage blocking layer having the first conductivity type and a firstnet doping concentration; a blocking junction on the voltage blockinglayer; and a plurality of discrete doped regions in the voltage blockinglayer and spaced apart from the blocking junction, wherein the pluralityof discrete doped regions have the first conductivity type and a secondnet doping concentration that is higher than the first net dopingconcentration, and wherein the plurality of discrete doped regions areconfigured to modulate the capacitance of the varactor diode as adepletion region of the varactor diode expands in response to a reversebias voltage applied to the blocking junction.
 2. The varactor diode ofclaim 1, wherein the plurality of discrete doped regions comprises firstdiscrete doped regions spaced a first distance from the blockingjunction and second discrete doped regions spaced a second distance fromthe carrier injection junction, wherein the second distance is greaterthan the first distance.
 3. The varactor diode of claim 2, wherein thefirst distance is between about 0.1 and 0.2 μm and wherein the seconddistance is between about 0.2 and 0.3 μm.
 4. The varactor diode of claim2, wherein the first discrete doped regions provide a first total chargeat the first distance from the blocking junction and the second discretedoped regions provide a second total charge at the second distance fromthe carrier injection junction, wherein the second total charge isgreater than the first total charge.
 5. The varactor diode of claim 2,wherein the first discrete doped regions have a first net dopingconcentration and the second discrete doped regions have a second netdoping concentration, wherein the second net doping concentration isless than the first net doping concentration.
 6. The varactor diode ofclaim 5, wherein the plurality of discrete doped regions furthercomprises third discrete doped regions spaced a third distance from thecarrier injection junction, wherein the third distance is greater thanthe second distance, and wherein the third discrete doped regions have athird net doping concentration, wherein the third net dopingconcentration is less than the second net doping concentration.
 7. Thevaractor diode of claim 2, wherein the plurality of discrete dopedregions further comprises third discrete doped regions spaced a thirddistance from the carrier injection junction, wherein the third distanceis greater than the second distance.
 8. The varactor diode of claim 7,wherein the third distance is between about 0.3 and 0.4 μm
 9. Thevaractor diode of claim 7, wherein the first, second and third discretedoped regions do not overlap in a lateral direction parallel to thecarrier injection junction.
 10. The varactor diode of claim 7, whereinthe thickness of the voltage blocking layer is not greater than twicethe total depth of the implanted doped regions.
 11. The varactor diodeof claim 1, further comprising a Schottky metal contact on the voltageblocking layer, wherein the blocking junction comprises a Schottkybarrier junction.
 12. The varactor diode of claim 1, further comprisinga second layer having a second conductivity type opposite the firstconductivity type on the voltage blocking layer, wherein the blockingjunction comprises a P-N junction between the second layer and thevoltage blocking layer.
 13. The varactor diode of claim 1, wherein thevoltage blocking layer comprises silicon carbide.
 14. A variableimpedance matching circuit for a high frequency amplifier comprising avaractor diode as recited in claim
 1. 15. A method of forming a varactordiode, comprising: providing a voltage blocking layer having a firstconductivity type and a first net doping concentration; forming aplurality of discrete doped regions in the voltage blocking layer andspaced apart from the carrier injection junction, wherein the pluralityof discrete doped regions have the first conductivity type and a secondnet doping concentration that is higher than the first net dopingconcentration, and wherein the plurality of discrete doped regions areconfigured to modulate the capacitance of the varactor diode as adepletion region of the varactor diode expands in response to a reversebias voltage applied to the carrier injection junction; and forming ablocking junction on the voltage blocking layer.
 16. The method of claim15, wherein forming the plurality of discrete doped regions comprisesimplanting ions into the voltage blocking layer.
 17. The method of claim16, wherein forming the plurality of discrete doped regions comprises:selectively implanting first ions at a first implant energy and firstdose to form first discrete doped regions spaced a first distance fromthe blocking junction and selectively implanting second ions at a secondimplant energy and second dose to form second discrete doped regionsspaced a second distance from the carrier injection junction, whereinthe second distance is greater than the first distance.
 18. The methodof claim 17, wherein selectively implanting first ions and second ionscomprises selectively implanting the first ions and the second ions sothat the first and second discrete doped regions do not overlap in alateral direction parallel to the carrier injection junction.
 19. Themethod of claim 17, wherein the first discrete doped regions provide afirst total charge at the first distance from the blocking junction andthe second discrete doped regions provide a second total charge at thesecond distance from the carrier injection junction, wherein the secondtotal charge is less than the first total charge.
 20. The method ofclaim 17, wherein the first discrete doped regions have a first netdoping concentration and the second discrete doped regions have a secondnet doping concentration, wherein the second net doping concentration isless than the first net doping concentration.
 21. The method of claim16, wherein implanting ions into the voltage blocking layer comprisesforming a multi-level implant mask on the voltage blocking layer andimplanting ions into the voltage blocking layer through themulti-layered implant mask wherein ions implanted through themulti-level implant mask are positioned at different depths in thevoltage blocking layer corresponding to different thicknesses of themulti-level implant mask.
 22. A varactor diode structure, comprising: acommon contact layer having a first conductivity type; a first voltageblocking layer having the first conductivity type and a first net dopingconcentration on the common contact layer; a first blocking junction onthe first voltage blocking layer; and a first plurality of discretedoped regions in the first voltage blocking layer and spaced apart fromthe first blocking junction, wherein the first plurality of discretedoped regions have the first conductivity type and a second net dopingconcentration that is higher than the first net doping concentration; asecond voltage blocking layer having the first conductivity type and athird net doping concentration on the common contact layer and spacedapart from the first voltage blocking layer; a second blocking junctionon the second voltage blocking layer; and a second plurality of discretedoped regions in the second voltage blocking layer and spaced apart fromthe second blocking junction, wherein the second plurality of discretedoped regions have the first conductivity type and a fourth net dopingconcentration that is higher than the third net doping concentration;and an ohmic contact on the common contact layer.